Catalytic steam cracking (CSC) of Tatar heavy oil has been studied using nanodispersed catalysts forming in situ based on K, Fe, Ni, Mo as well as several Mo-based catalysts promoted by Ni-, Co-, and Al-based additives. The upgrading was carried out at 425 °C, catalyst content of 2 wt%, water to heavy oil wt. ratio of 0.3:1, and 1 h of residence time using a batch reactor. The highest upgrading efficacy was found in the case of non-promoted Mo-based catalyst: at a low coke yield the upgraded oil had the lowest S content as well as the highest H:С ratio. The participation of water in the case of Mo- and Ni-dispersed catalysts was confirmed by comparison with the water-free cracking experiments at the same conditions. XRD and TEM characterization have shown that the active components were in a form of oxides and/or sulfides.